In the PUREX process ("PUREX" is an acronym for "plutonium-uranium recovery by extraction"), waste reprocessing facilities dissolve used fuel from nuclear reactors in nitric acid. The uranium and plutonium are extracted with an organic solvent and the remaining aqueous phase is frequently neutralized with sodium hydroxide to permit storage in carbon steel tanks.
Because this PUREX waste is radioactive, yet has no commercial utility, it must be safely disposed of such as by immobilization in glass at a vitrification facility. The quantity of PUREX waste at some waste reprocessing facilities is too small to justify the cost of constructing a vitrification plant at the reprocessing facility, which means that the PUREX waste must be transported to a central vitrification facility. However, because the PUREX waste is a radioactive liquid it cannot be transported due to the danger of spillage in route. Evaporation of the water in the PUREX waste would produce a fine powder which also cannot be transported because of the danger that any container in which the powder is placed may break open, permitting the wind to disperse the powder. Thus, the waste can only be transported in the form of a solid having a particle size large enough to prevent air dispersion.